1. Field of the Invention
The present invention relates to a water-cooled type internal combustion engine including a cylinder provided in a cylinder block of an engine body and having a piston slidably received therein, and a cooling water passageway defined in the engine body and surrounding the cylinder, and particularly, to an improvement in the structure of such a water-cooled type internal combustion engine, w her e in a piston slap sound produced by the collision of the piston against an inner surface of the cylinder is reduced.
2. Description of the Related Art
In reducing the piston slap sound in the cylinder block structure in the water-cooled type internal combustion engine, at least the five following techniques have been conventionally employed. First (1), a technique in which the wall thickness of the cylinder is increased to suppress the amplitude of a vibration to a small level. Second (2), a technique in which the wall thickness of an outer wall of the cylinder block is increased to suppress the amplitude of a vibration. Also known are three structures designed to inhibit the vibration of the non-compressible cooling water existing in the cooling water passageway. Third (3), a structure in which an expandable member such as a gas-encapsulated bellows is mounted in the outer wall of the cylinder block in such a manner that it is disposed in the cooling water passageway, as disclosed in Japanese Utility Model Application Laid-open No.57-101345. Fourth (4), a structure in which a sound shielding layer is provided in the cylinder block outside the cooling water passageway with a partition wall interposed therebetween, as disclosed in Japanese Utility Model Application Laid-open No.53-68814. Fifth (5), a structure in which a sponge-like damper material covered with a metal plate is affixed to an inner surface of the outer wall of the cylinder block in the cooling water passageway, as disclosed in Japanese Patent Application Laid-open No.57-102539.
However, in the techniques (1) and (2), the weight of the engine body is increased due to increases in wall thickness of the cylinder and the cylinder block. In the structure (3), the existence of the expandable member in the cooling water passageway causes the flow of the cooling water in the cooling water passageway to be hindered, bringing about a reduction in cooling performance, and also the spring characteristic of the expandable member is varied in accordance with a variation in internal pressure of a gas in the expandable member depending upon the temperature of the cooling water, thereby reducing the vibration damping effect by half during operation of the engine. In the structure (4), the cooling water passageway and the sound shielding layer are disposed with the partition wall interposed therebetween to provide a double structure and hence, this structure is complicated and difficult to manufacture, resulting in an increase in manufacture cost, and bringing about an increase in weight of the engine body. Further, in the structure (5), the presence of the damper material covered with the metal plate in the cooling water passageway causes the flow of the cooling water in the cooling water passageway to be hindered, bringing about a reduction in cooling performance.